Mercury is an undesirable impurity found in many petrochemical process streams and in much of the natural gas found throughout the world. The mercury impurity present in such streams is generally in the form of elemental mercury, but in some instances the mercury is in the form of mercury compounds, including organic mercury compounds. Mercury impurities in process streams, particularly light hydrocarbon streams, that is, wherein the light hydrocarbons comprise methane (C.sub.1) through C.sub.10 hydrocarbons, may cause in corrosion problems in process equipment or poison sensitive downstream catalytic processes. Consequently, a considerable number of methods and schemes have been devised to selectively remove the mercury. The purification processes are most often based on adsorption technology wherein the mercury is selectively adsorbed on to the adsorbent. Some of these processes involve the use of non-regenerable adsorbents, but technology based on non-regenerable adsorbents usually results in the production of a solid adsorbent loaded with mercury and thus presents a hazardous waste disposal problem. The most commonly used adsorbent for the removal of mercury is an activated carbon as a support for a mercury reactive material such as potassium triodide, sulfur, sulfuric acid, chlorine, silver, copper, or various salts of silver or copper. Other supports for mercury reactive materials include silicas, aluminas, silica-aluminas, and zeolitic aluminosilicates. Ion-exchange resins, particularly the strong basic anion-exchange types which have been reacted with a polysulfide, have also been reported as useful mercury adsorbents. See U.S. Pat. No. 4,591,490 (Horton) in this latter regard. The disclosures of U.S. Pat. No. 4,500,327 (Nishino) and U.S. Pat. No. 4,196,173 (de Jong et al.) are relevant to the use of activated carbon support for mercury reactive materials. U.S. Pat. No. 5,523,067 relates to processing both gas and liquid hydrocarbon streams containing mercury.
U.S. Pat. No. 5,281, 258 to Markovs discloses a process for removing mercury vapor from a natural gas stream which comprises mercury and water. The natural gas stream is passed through a first fixed bed adsorber containing a regenerable adsorbent which adsorbs mercury and water and a purified effluent is recovered. The flow of the natural gas stream to the first adsorber bed is terminated and a heated purge desorbent stream is passed through the first adsorbent bed to desorb mercury and water to produce a spent regenerant. The spent regenerant is cooled and condensed to recover liquid mercury and water. The remainder of the spent regenerant is passed to a second fixed bed adsorber containing a regenerable adsorbent with a strong affinity for adsorbing water to produce a second effluent, decreased in water. The second effluent is cooled and condensed to condense out a portion of the mercury from the second effluent. The second fixed bed adsorber is regenerated with a portion of the heated purge desorbent and is not recovered. The second fixed bed adsorber is required to remove water prior to the condensing out of the mercury to prevent hydrate formation.
U.S. Pat. No. 5,281,259 to Markovs discloses a process for the removal of mercury from a natural gas stream wherein the mercury vapor contained in the purge gas used to regenerate the adsorption beds is recovered as liquid mercury. In this scheme, a primary spent purge desorbent from a primary bed undergoing desorption is cooled and condensed to recover mercury and water and the remaining material is passed to a secondary bed containing a regenerable adsorbent for mercury to produce a second effluent stream depleted in mercury. Another secondary bed undergoing regeneration at the same time as the primary bed is purged with a portion of the purge desorbent to produce a secondary spent regenerant. The secondary spent regenerant is combined with the primary spent desorbent prior to the cooling and condensing step.
U.S. Pat. No. 5,271,760 to Markovs discloses a process for the removal of mercury from a process feedstream to recover liquid mercury. The process comprises the passing of the feedstream periodically in sequence through two fixed beds containing a regenerable adsorbent selective for the adsorption of mercury. Each of the beds cyclically undergoes an adsorption step wherein the feedstream is passed through the bed to selectively adsorb mercury and to produce an effluent stream, and a purge desorption step wherein the adsorbed mercury is desorbed by passing a regeneration fluid through the bed to produce a second effluent. The improvement comprises the tandem operation of the beds so that as one bed is operating in the adsorption step, the other bed is operating in the purge desorption step and the second effluent is cooled and condensed to recover a portion of the mercury. Markovs further discloses that the remainder of the second effluent is recombined with the feedstream and passed to the bed undergoing adsorption. The above U.S. Pat. Nos. 5,281,258, 5,281,259, and 5,271,760 are hereby incorporated by reference.
Perhaps the two greatest problems involved in removing mercury from process streams are (a) achieving a sufficient reduction in the mercury concentration of the feedstream being treated and (b) avoiding the reentry of the recovered mercury into some other environment medium. Although permissible levels of mercury impurity vary considerably, depending upon the ultimate intended use of the purified product, for purified natural gas, a mercury concentration greater than about 0.01 microgram per normal cubic meter (.mu.g/Nm.sup.3) is considered undesirable, particularly in those instances in which the natural gas is to be liquefied by cryogenic processing. To attain lower concentration levels requires the use of relatively large adsorption beds and relatively low mercury loading. If non-regenerable, the capital and adsorbent costs are uneconomical, and if regenerable, the regeneration media requirements are not only large, but also result in a large mercury-laden bed effluent which must itself be disposed of in an environmentally safe manner. Furthermore, the high volume of regeneration gas required to be first heated and then cooled to recover the mercury can result in oversized regeneration equipment which increases the capital and utility costs of the process installation.
Purification processes are sought for the efficient removal and recovery of mercury from hydrocarbon streams with a minimum of process equipment.
Purification processes are sought for the high recovery of mercury from both gaseous and liquid hydrocarbon streams.